1. Field of the Invention
The present invention relates to a system for controlled delivery of a gas from a liquified state, and to a semiconductor processing system comprising the same. The present invention also relates to a method for controlled delivery of a gas from a liquified state.
2. Description of the Related Art
In the semiconductor manufacturing industry, high purity gases stored in cylinders are supplied to process tools for carrying out various semiconductor fabrication processes. Examples of such processes include diffusion, chemical vapor deposition (CVD), etching, sputtering and ion implantation. The gas cylinders are typically housed within gas cabinets. These gas cabinets also contain means for safely connecting the cylinders to respective process gas lines via a manifold. The process gas lines provide a conduit for the gases to be introduced to the various process tools.
Of the numerous gases utilized in the semiconductor manufacturing processes, many are stored in cylinders in a liquified state. A partial list of chemicals stored in this manner, and the pressures under which they are stored, is provided below in Table 1:
TABLE 1 ______________________________________ Vapor Pressure of Gas at 20.degree. C. Chemical Formula (psia) ______________________________________ Ammonia NH.sub.3 129 Arsine AsH.sub.3 220 Boron Trichloride BCl.sub.3 19 Carbon Dioxide CO.sub.2 845 Chlorine Cl.sub.2 100 Dichlorosilane SiH.sub.2 Cl.sub.2 24 Disilane Si.sub.2 H.sub.6 48 Hydrogen Bromide HBr 335 Hydrogen Chloride HCl 628 Hydrogen Fluoride HF 16 Nitrous Oxide N.sub.2 O 760 Perfluoropropane C.sub.3 F.sub.8 115 Sulfur SF.sub.6 335 Hexafluoride Phosphine PH.sub.3 607 Tungsten WF.sub.6 16 Hexafluoride ______________________________________
The primary purpose of the gas cabinet is to provide a safe vehicle for delivering one or more gases from the cylinder to the process tool. The gas cabinet typically includes a gas panel with various flow control devices, valves, etc., in a configuration allowing cylinder changes and/or component replacement in a safe manner.
The cabinets conventionally include a system for purging the gas delivery system with an inert gas (e.g., nitrogen or argon) before breaking any seals. Control and automation of purging operations are known in the art, and are disclosed, for example, in U.S. Pat. No. 4,989,160, to Garrett et al. This patent indicates that different purging procedures are required for different types of gases, but does not recognize any special concerns with respect to liquified gas cylinders.
In the case of HCl, condensation occurs by the Joule-Thompson effect (see, Joule-Thompson Expansion and Corrosion in HCl System, Solid State Technology, July 1992, pp. 53-57). Liquid HCl is more corrosive than its vapor form. Likewise, for the majority of chemicals listed above in Table 1, the liquid forms thereof are more corrosive than their respective vapor forms. This is due to impurities, such as moisture, which are trapped in the liquid phase and which exist at surfaces of the gas distribution system. Thus, condensation of these materials in the gas delivery system can lead to corrosion, which is harmful to the components of the system. Furthermore, the corrosion products can lead to contamination of the highly pure process gases. This contamination can have deleterious effects on the processes being run, and ultimately on the manufactured semiconductor devices.
The presence of liquid in the gas delivery system has also been determined to lead to inaccuracies in flow control. That is, the accumulation of liquid in various flow control devices can cause flowrate and pressure control problems as well as component failure, leading to misprocessing. One example of such behavior is the swelling of a valve seat by liquid chlorine, which causes the valve to become permanently closed.
In typical gas delivery systems, the first component through which the gas passes after leaving the cylinder is a pressure reduction device, such as a pressure regulator or orifice. However, for cylinders containing materials with relatively low vapor pressures (e.g., WF.sub.6, BCl.sub.3, HF and SiH.sub.2 Cl.sub.2), a regulator may not be suitable, in which case the first component can be a valve. These regulators or valves often fail during service and require replacement. The failure of such components can often be attributed to the presence of liquid in the components. Such failure can necessitate shutdown of the process during replacement of the failed parts and subsequent leak checking. Extensive process downtime can result.
In U.S. Pat. No. 5,359,787, to Mostowy, Jr. et al, an apparatus is described for the delivery of hygroscopic, corrosive chemicals such as HCl from a bulk source (e.g., a tube trailer) to a point of use. This patent discloses use of an inert gas purge and vacuum cycle, and a heated purifier downstream of the bulk storage container. By heating during pressure reduction, condensation of the corrosive gas is prevented in the delivery line. U.S. Pat. No. 5,359,787 is directed to bulk storage systems in which the volumes of stored chemicals are substantially larger than the volumes typical of cylinders stored in gas cabinets. As a result of the large volumes associated with bulk storage systems, temperature and pressure within bulk storage containers are generally constant until the liquid in the container becomes substantially depleted. Pressure in such containers is primarily controlled by seasonal variations in the ambient temperature.
In contrast, variations in pressure of the comparatively low volume cylinders stored in gas cabinets depend upon the rate of gas withdrawal from the cylinder (and the removal of the necessary heat of vaporization) as well as the transfer of ambient energy to the cylinder. Such effects are not typically present in bulk storage systems. In bulk storage systems, the thermal mass of the stored chemical is sufficiently large that liquid temperature variation occurs relatively slowly. Gas pressure in bulk systems is controlled by the temperature of the liquid. That is, the pressure inside the container is equal to the vapor pressure of the chemical at the temperature of the liquid contained therein. In gas delivery systems based on cylinders, the need to control cylinder pressure by controlling liquid temperature vis-a-vis cylinder temperature is recognized in the art. Gas cylinder heating/cooling jackets have been proposed for controlling cylinder pressure through the control of cylinder temperature. In such a case, a heating/cooling jacket can be placed in intimate contact with the gas cylinder. The jacket is maintained at a constant temperature by a circulating fluid, the temperature of which is controlled by an external heater/chiller unit. Such heating/cooling jackets are commercially available, for example, from Accurate Gas Control Systems, Inc.
These heating/cooling jackets are typically used for controlling the temperature of thermally unstable gases, such as diborane (B.sub.2 H.sub.6). Another use for the heating/cooling jackets is in the heating of cylinders containing low vapor pressure gases such as BCl.sub.3, WF.sub.6, HF and SiH.sub.2 Cl.sub.2. Because the cylinder pressure for these gases is low, any further decrease in pressure due to a lowering of the liquid temperature can create flow control problems.
Control of cylinder temperature coupled with thermal regulation of the entire gas piping system to prevent recondensation in the gas delivery system has also been proposed for gases having low vapor pressures. The requirement for thermal regulation of the piping system is a result of the greater than ambient temperature of the cylinder caused by the heating/cooling jacket. If the gas line is not thermally controlled, recondensation of the gas flowing therethrough can occur when it passes from the heated zone into a lower temperature zone. Heating/cooling jackets coupled with thermal regulation is not favored, however, due to the complications associated with system maintenance (e.g., during cylinder replacement) and the added expense. In addition, heating/cooling jackets have great potential for overheating since the jackets are wrapped around the cylinder, since the entire system is heated and brought to the heating temperature. Such overheating can result in recondensation in the gas distribution system downstream of the cylinder, resulting from the lower temperatures. As a result, heating of the entire distribution system from the gas cylinder to the point-of-use becomes necessary to prevent such recondensation.
Moreover, cylinder heating/cooling jackets are not thermally efficient. For example, typical cylinder heating/cooling jackets have heating and cooling capabilities of about 1500 W. Table 2 summarizes the energy requirements for the continuous vaporization of various gases at flowrates of 10 slm from a cylinder. This data demonstrates that the energy requirements for vaporization are substantially less than the heating/cooling ratings of the cylinder jackets.
TABLE 2 ______________________________________ Energy Energy required for required for Chemical 10 slm (W) Chemical 10 slm (W) ______________________________________ Ammonia 133.8 Hydrogen 61.8 Chloride Arsine 115.1 Hydrogen 60 Fluoride Boron 156.4 Nitrous Oxide 55.7 Trichloride Chlorine 122.4 Perfluoropro- 111.5 pane Dichlorosilane 153.2 Sulfur 107.7 Hexafluoride Hydrogen 85.7 Tungsten 179 Bromide Hexafluoride ______________________________________
The above described disadvantages associated with the use of heating/cooling jackets and strict thermal regulation of gas distribution systems make use thereof undesirable.
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel system for controlled delivery of gases from a liquified state which will allow for accurate control of the pressure in cylinders containing liquified gases, while simultaneously minimizing entrained droplets in the gases withdrawn from the cylinders. Thus, single phase process gas flow can be obtained with a substantially increased flowrate. As a result, a number of process tools can be serviced by a single gas cabinet. Alternatively, a higher flowrate can be delivered to an individual process tool. Moreover, use of cumbersome heating/cooling jackets and strict thermal management of the process line can be avoided.
It is a further object of the present invention to provide a semiconductor processing system which comprises the inventive system for controlled delivery of gases from a liquified state.
It is a further object of the present invention to provide a method for controlled delivery of gases from a liquified state.
It is a further object of the present invention to provide a heated valve for regulating the flow of a gas, which can be used in conjunction with the inventive system and method.
It is a further object of the present invention to provide a heated scale cover which can be used in the inventive system and method.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.